26,631 research outputs found
Thermal DMRG for highly frustrated quantum spin chains: a user perspective
Thermal DMRG is investigated with emphasis of employability in molecular
magnetism studies. To this end magnetic observables at finite temperature are
evaluated for two one-dimensional quantum spin systems: a Heisenberg chain with
nearest-neighbor antiferromagnetic interaction and a frustrated sawtooth
(delta) chain. It is found that thermal DMRG indeed accurately approximates
magnetic observables for the chain as well as for the sawtooth chain, but in
the latter case only for sufficiently high temperatures. We speculate that the
reason is due to the peculiar structure of the low-energy spectrum of the
sawtooth chain induced by frustration.Comment: 18 pages, 5 figure
Recursion method and one-hole spectral function of the Majumdar-Ghosh model
We consider the application of the recursion method to the calculation of
one-particle Green's functions for strongly correlated systems and propose a
new way how to extract the information about the infinite system from the exact
diagonalisation of small clusters. Comparing the results for several cluster
sizes allows us to establish those Lanczos coefficients that are not affected
by the finite size effects and provide the information about the Green's
function of the macroscopic system. The analysis of this 'bulk-related' subset
of coefficients supplemented by alternative analytic approaches allows to infer
their asymptotic behaviour and to propose an approximate analytical form for
the 'terminator' of the Green's function continued fraction expansion for the
infinite system. As a result, the Green's function acquires the branch cut
singularity corresponding to the incoherent part of the spectrum. The method is
applied to the spectral function of one-hole in the Majumdar-Ghosh model (the
one-dimensional model at ). For this model,
the branch cut starts at finite energy , but there is no upper bound
of the spectrum, corresponding to a linear increase of the recursion
coefficients. Further characteristics of the spectral function are band gaps in
the middle of the band and bound states below or within the gaps.
The band gaps arise due to the period doubling of the unit cell and show up as
characteristic oscillations of the recursion coefficients on top of the linear
increase.Comment: 12 pages, 7 figure
Quantum -- antiferromagnet on the stacked square lattice: Influence of the interlayer coupling on the ground-state magnetic ordering
Using the coupled-cluster method (CCM) and the rotation-invariant Green's
function method (RGM), we study the influence of the interlayer coupling
on the magnetic ordering in the ground state of the spin-1/2
- frustrated Heisenberg antiferromagnet (- model) on the
stacked square lattice. In agreement with known results for the -
model on the strictly two-dimensional square lattice () we find that
the phases with magnetic long-range order at small and large
are separated by a magnetically disordered (quantum
paramagnetic) ground-state phase. Increasing the interlayer coupling
the parameter region of this phase decreases, and, finally, the
quantum paramagnetic phase disappears for quite small .Comment: 4 pages, 3 figure
Influence of the spin quantum number on the zero-temperature phase transition in the square lattice - model
We investigate the phase diagram of the Heisenberg antiferromagnet on the
square lattice with two different nearest-neighbor bonds and (-
model) at zero temperature. The model exhibits a quantum phase transition at a
critical value between a semi-classically ordered N\'eel and a
magnetically disordered quantum paramagnetic phase of valence-bond type, which
is driven by local singlet formation on bonds. We study the influence of
spin quantum number on this phase transition by means of a variational
mean-field approach, the coupled cluster method, and the Lanczos
exact-diagonalization technique. We present evidence that the critical value
increases with growing according to .Comment: 13 pages, 6 figure
The spin-1/2 square-lattice J_1-J_2 model: The spin-gap issue
We use the coupled cluster method to high orders of approximation in order to
calculate the ground-state energy, the ground-state magnetic order parameter,
and the spin gap of the spin-1/2 J_1-J_2 model on the square lattice. We obtain
values for the transition points to the magnetically disordered quantum
paramagnetic phase of J_2^{c1}=0.454J_1 and J_2^{c2}= 0.588 J_1. The spin gap
is zero in the entire parameter region accessible by our approach, i.e. for J_2
\le 0.49J_1 and J_2 > 0.58J_1. This finding is in favor of a gapless
spin-liquid ground state in this parameter regime.Comment: 10 pages, 3 figures, accepted versio
The frustrated spin-1/2 J1-J2 Heisenberg ferromagnet on the square lattice: Exact diagonalization and Coupled-Cluster study
We investigate the ground-state magnetic order of the spin-1/2 J1-J2
Heisenberg model on the square lattice with ferromagnetic nearest-neighbor
exchange J1<0 and frustrating antiferromagnetic next-nearest neighbor exchange
J2>0. We use the coupled-cluster method to high orders of approximation and
Lanczos exact diagonalization of finite lattices of up to N=40 sites in order
to calculate the ground-state energy, the spin-spin correlation functions, and
the magnetic order parameter. We find that the transition point at which the
ferromagnetic ground state disappears is given by J2^{c1}=0.393|J1| (exact
diagonalization) and J2^{c1}=0.394|J1| (coupled-cluster method). We compare our
results for ferromagnetic J1 with established results for the spin-1/2 J1-J2
Heisenberg model with antiferromagnetic J1. We find that both models (i.e.,
ferro- and antiferromagnetic J1) behave similarly for large J2, although
significant differences between them are observed for J2/|J1| \lesssim 0.6.
Although the semiclassical collinear magnetic long-range order breaks down at
J2^{c2} \approx 0.6J1 for antiferromagnetic J1, we do not find a similar
breakdown of this kind of long-range order until J2 \sim 0.4|J1| for the model
with ferromagnetic J1. Unlike the case for antiferromagnetic J1, if an
intermediate disordered phase does occur between the phases exhibiting
semiclassical collinear stripe order and ferromagnetic order for ferromagnetic
J1 then it is likely to be over a very small range below J2 \sim 0.4|J1|.Comment: 15 pages, 7 figures, 2 table
Direct calculation of the spin stiffness on square, triangular and cubic lattices using the coupled cluster method
We present a method for the direct calculation of the spin stiffness by means
of the coupled cluster method. For the spin-half Heisenberg antiferromagnet on
the square, the triangular and the cubic lattices we calculate the stiffness in
high orders of approximation. For the square and the cubic lattices our results
are in very good agreement with the best results available in the literature.
For the triangular lattice our result is more precise than any other result
obtained so far by other approximate method.Comment: 5 pages, 2 figure
High-Order Coupled Cluster Calculations Via Parallel Processing: An Illustration For CaVO
The coupled cluster method (CCM) is a method of quantum many-body theory that
may provide accurate results for the ground-state properties of lattice quantum
spin systems even in the presence of strong frustration and for lattices of
arbitrary spatial dimensionality. Here we present a significant extension of
the method by introducing a new approach that allows an efficient
parallelization of computer codes that carry out ``high-order'' CCM
calculations. We find that we are able to extend such CCM calculations by an
order of magnitude higher than ever before utilized in a high-order CCM
calculation for an antiferromagnet. Furthermore, we use only a relatively
modest number of processors, namely, eight. Such very high-order CCM
calculations are possible {\it only} by using such a parallelized approach. An
illustration of the new approach is presented for the ground-state properties
of a highly frustrated two-dimensional magnetic material, CaVO. Our
best results for the ground-state energy and sublattice magnetization for the
pure nearest-neighbor model are given by and ,
respectively, and we predict that there is no N\'eel ordering in the region
. These results are shown to be in excellent agreement
with the best results of other approximate methods.Comment: 4 page
- …